Field of the Invention
The invention relates to a unit for outputting a signal on a transmission channel in a motor vehicle, a unit for receiving a signal from a transmission channel in a motor vehicle, an arrangement for data transmission in a motor vehicle via a transmission channel and a method for transmitting and receiving data in a vehicle.
Motor vehicles often have distributed control or computing units. Distributed control or computing units of this sort are generally understood to include units that are arranged in different locations in the motor vehicle. Because of their need to exchange data, these control and computing units are interconnected via a non-contact or wire-bound transmission channel whereby sensors arranged in the engine compartment, in the doors or in the tires, for example, transmit data to central computing units that algorithmically evaluate the received data and control actuators accordingly.
The wire-bound networking of control or computing units with sensor units is usually implemented by means of a bus system. An example of such a bus system is the well-known CAN-bus (CAN=Controller Area Network). Special transmitter and receiver devices or driver modules, in particular transceivers, are provided as means of accessing the bus transmission channel. Different speeds are used depending on the application: a high-speed CAN with data rates of 500 kbps for the drive train in the vehicle, for example, and the low-speed CAN with data rates of below 125 kbps, for example 83 kbps, for the car body area.
If the vehicle is involved in a traffic accident the CAN transmission channel may be damaged and short-circuited. This is also referred to as external short-circuiting of the bus lines (Bus_L; Bus_H) of the transmission channel. Four different scenarios are possible: Bus_L to chassis or earth (GND); Bus_L to battery (Vbat or BAT) with a voltage of 12V currently, and 42V soon; Bus_H to GND and Bus_H to BAT. Unlike in the low-speed area, in the high-speed area there are currently no fault-tolerant driver modules available, or—at best—only two of the above scenarios of external short-circuiting on one of the bus lines in the transmission channel are tolerated; these are either Bus_H to BAT or short-circuiting of the Bus_L line to GND. The drawback of this is that the transmission of sensor signals is not guaranteed at the very times when it could be vital, in particular if it involves sensor signals of a passive security system such as an airbag system, seatbelt tensioning system or similar.
Data transmission between the aforementioned units usually takes place asynchronously. To ensure that the data is correctly reconstructed in the receiver, therefore, said receiver must know the clock pulse data of the transmitting unit. This clock pulse data must therefore be transmitted from the transmitter to the receiver via the transmission route. If clock pulse data is transmitted in addition to other information, the transmission bandwidth is increased. The data transmission has an overhead.
The data generated in a unit—for example a sensor—is coded in this unit for transmission to a remote location. This is also referred to by experts as channel coding, which converts the data generated into a form suitable for transmission. This is effected on the basis of a coding rule which converts the sensor signal into the signal to be transmitted. The term sensor signal is used throughout the following to mean the signal present in the transmitter, of which the information is to be transmitted to the receiver.
In motor vehicle technology such sensor signals are usually coded according to the NRZ Code or the Manchester Code and then transmitted. In this connection, FIG. 5 shows these known coding processes for data transmission in a motor vehicle.
FIG. 5a shows a sensor signal DATA over the time t, the information of which is to be transmitted to a receiver via a transmission channel. This sensor signal DATA is binary-coded and therefore has a character set of two characters: a “0” and a “1”. Individual signal units of the sensor signal DATA have a duration T. Several such signal units arranged in sequence and each occupied by a character from the character set, together produce a data word which is physically present as a signal, characterized by its voltage or power states. FIG. 5b shows an power cycle TAKT of the transmitting output unit over the time t.
FIGS. 5c and 5d show signals to be transmitted CHAN, which pertain to the sensor signal DATA and are channel coded according to defined coding rules, whereby FIG. 5c shows a signal to be transmitted CHAN which has been recovered from the sensor signal DATA following NRZ coding. This coding is initially a 1:1 mapping of the sensor signal. In the UART (Universal Asynchronous Receive Transmit) standard the receiver is synchronized only by a start signal. The free-running oscillator provided in the receiver for clock generation must not abandon a predefined tolerance range until further synchronization with the transmitter is effected by a further start signal. This requires either a highly accurate oscillator in the receiver unit, or very frequent synchronization, which can place a burden on transmission bandwidths.
FIG. 5d shows a channel-coded signal to be transmitted CHAN, which has been recovered from the sensor signal DATA by Manchester coding. A feature of Manchester coding is that, like NRZ coding, it uses a binary character set. In the Manchester-coded signal, however, two characters/signal states are provided within a signal time unit T of the sensor signal. The change from one character in the sensor signal to its complementary character in the subsequent signal state is implemented in the Manchester code by a phase change. The Manchester Code therefore does offer the possibility of clock recovery in the receiver within a theoretical tolerance range of 50%. However, this timing recovery is at the expense of a doubling in the bandwidth, since one signal unit (bit) of the sensor signal is represented by two signal states during the same period of time T in the signal to be transmitted.
WO 98/52 792—A discloses a channel coding based on current modulation. In this method the channel coding has a character set of three characters, HIGH, LOW and zero. The sensor signal provides a binary code. According to the coding rule, the “ones” in the sensor signal are coded alternately into HIGH and LOW pulses in the signal to be transmitted. ZEROs in the sensor signal remain at ZERO level in the signal to be transmitted.
In this known data transmission method the temporal average of the signals to be transmitted is kept constant. However, a power cycle cannot be recovered from the signal to be transmitted.
EP 0 384 258 A2 discloses a data transmission method in which a binary sensor signal is channel-coded by means of an AMI (Alternate Mark Inversion) code combined with a pulse width modulation. In this method, the sensor signal undergoes pulse width modulation first and the pulse width-modulated signal thus formed is then subjected to Alternate Mark Inversion.
The drawback of this data transmission method is the increased bandwidth in the signal to be transmitted compared to the sensor signal. Moreover, the narrow pulses generated by pulse width modulation create problems in terms of electromagnetic compatibility (EMC) auf.
To alleviate these problems, DE 101 32 048 proposes designing a channel coding of the type whereby the code for the signal to be transmitted via the channel contains at least one more character in its character set than the character set from which the sensor signal is formed, of which the information is ultimately to be transmitted. Thus, for example, a binary code may be provided for the sensor signal, and then the signal to be transmitted is formed at least from a ternary code, i.e. three different characters—which are, for example, reflected by three different signal states on the line—are available for forming a signal. In general, n characters are available for the sensor signal, with n being a whole number, and at least n+1 characters are available for the signal to be transmitted.